An organic EL element is a self-luminous element in which a light emitting layer made of an organic compound is provided between a cathode and an anode that are opposite to each other. When a voltage is applied between the cathode and the anode, light is emitted by excitations which are generated by the recombination of electrons which are injected from the cathode to the light emitting layer and holes which are injected from the anode to the light emitting layer in the light emitting layer.
Tang et al. in Eastman Kodak Company succeeded in developing the following structure in order to improve the efficiency of an organic EL element: organic compounds with different carrier transport properties were laminated such that electrons and holes were respectively injected from a cathode and an anode with a good balance therebetween; the thickness of an organic layer interposed between the cathode and the anode was equal to or less than 2000 {acute over (Å)}; and when a voltage of 10 V or less was applied, high brightness and efficiency sufficient to practically obtain 1000 cd/m2 and an external quantum efficiency of 1% was obtained (see NPL 1).
According to the patent applied for by Tang et al., the overall thickness of the organic layer interposed between the cathode and the anode is equal to or less than 1 μm. Therefore, it is possible to provide a device which can emit light at a low applied voltage. Preferably, when the thickness of the organic layer is in the range of 1000 {acute over (Å)} to 5000 {acute over (Å)}, it is possible to obtain an electric field (V/cm) that is useful to emit light at an applied voltage of 25 V or less (see PTL 1 to PTL 6).
The organic EL element has been developed on the basis of the element structure disclosed by Tang et al.
As the element structure of the organic EL element, an organic EL element with a tandem structure in which a plurality of light emitting units, each of which is a unit including at least one light emitting layer, are laminated between a cathode and an anode so as to be connected in series to each other has been developed (see PTL 7 and PTL 8).
The organic EL element with the tandem structure has drawn attention as a technique which can achieve a long lifespan, high brightness, and uniform light emission from a large area which have not been achieved by the structure of the organic EL element developed by Tang et al. which requires a large amount of current even though the low voltage is applied.
In addition, an organic EL element with a multi-photon emission (MPE) structure has been developed in which an electrically insulating charge generation layer (CGL) is provided between a plurality of light emitting units (see PTL 9 and PTL 10). In the organic EL element with the MPE structure, when a voltage is applied between a cathode and an anode, charges in a charge-transfer complex are moved to the cathode and the anode. Holes are injected into one light emitting unit which is disposed close to the cathode, with respect to the charge generation layer interposed between one light emitting unit and the other light emitting unit, and electrons are injected into the other light emitting unit which is disposed close to the anode, with respect to the charge generation layer interposed between one light emitting unit and the other light emitting unit. In this way, light is simultaneously emitted from the plurality of light emitting units while the amount of current is maintained. Therefore, it is possible to obtain current efficiency and external quantum efficiency corresponding to the multiple of the number of light emitting units.
The organic EL element can switch light emission at high speed and has a simple element structure and a small thickness. Since the organic EL element has the above-mentioned excellent characteristics, it is applied to, for example, a display device of a mobile phone or an in-vehicle device. In recent years, the organic EL element has drawn attention, for example, as a backlight of a liquid crystal display or a lighting device for general lighting since it has thin surface emission characteristics.
However, when the organic EL element is applied to a lighting device, it is preferable to obtain white light with excellent color rendering properties. As a method for obtaining white light from the organic EL element, there are the following methods: a method which mixes light emitted from two light emitting materials of complementary colors, for example, a blue light emitting layer and a yellow light emitting layer to obtain white light; and a method which mixes light emitted from three (red, blue, and green) light emitting layers to obtain white light.
Among the methods, the method which mixes light emitted from three (red, blue, and green) light emitting layers is suitable for obtaining white light with high color rendering properties (see PTL 11 and PTL 12).
However, in the organic EL element according to the related art, the emission spectrum of the white light emitted from the three (red, blue, green) light emitting layers is not sufficiently controlled in order to obtain high color rendering properties. That is, the organic EL element according to the related art does not have a sufficient performance as a light source of a lighting device.